Stem cells are self-renewing, pluripotent cells that can give rise to multiple cell lineages and/or cell types of the body. The development of human and mouse embryonic stem cells and the isolation of adult stem cells have received much attention. The possibility of using stem/progenitor cells to replace damaged or diseased human tissue has been widely publicized. One application of this technology would be to repair damaged cardiac and vascular tissue; however, this will first require identification and characterization of vascular stem/progenitor cells. The goal of this project is to identify and characterize vascular stem/progenitor cells and to explore their developmental potential in vitro and in vivo. We will mark early vascular progenitor cells with an autofluorescent reporter suitable for flow cytometry, such as enhanced green fluorescent protein (EGFP). We will "knock-in" the auto fluorescent tag into the endogenous locus of early endothelial genes in mouse embryonic stem (ES) cells. Second, we will isolate and characterize marked progenitor populations from differentiating ES cells and embryonic yolk sac membranes. Third, we will examine the lineage differentiation potential of these progenitor cells in vitro and in vivo. We anticipate that this project will provide important insights into the nature of stem cells at the beginning of the vascular system, and possibly common to the hematopoietic and endothelial lineages. Ultimately, these studies will address basic questions of using stem cells for damaged or diseased blood vessels.